1. Field of the Invention
The invention relates to an illumination system for a microlithographic projection exposure apparatus and in particular to an illumination system that comprises a polarizer. The invention further relates to polarizers suitable for such illumination systems.
2. Description of the Related Art
Integrated electrical circuits and other microstructured components are usually manufactured by applying a plurality of structured layers to a suitable substrate which may be, for example, a silicon wafer. In order to structure the layers they are first covered with a resist which is sensitive to light of a given wavelength range, e.g. light in the deep ultraviolet (DUV) spectral range. The wafer coated in this way is then exposed in a projection exposure apparatus. A pattern of diffractive structures arranged on a reticle is imaged on the resist by means of a projection lens. Because the imaging scale is generally less than 1:1, such projection lenses are frequently also referred to as reduction lenses.
After the resist has been developed, the wafer is subjected to an etching process whereby the layer is structured according to the pattern contained in the reticle. The remaining resist is then removed from the remaining parts of the layer. This process is repeated until all the layers have been applied to the wafer.
In important part of a projection exposure apparatus used for the exposure is an illumination system which generates a projection light beam and directs it onto the reticle. The illumination system generally includes a laser as a light source that generates linearly polarized light. Especially with the use of very short-wave projection light (e.g. λ=157 nm), however, undesired polarization-dependent effects which disturb the imaging properties of the projection lens may occur with certain optical elements. An example of such disturbing factors is the intrinsic birefringence of calcium fluoride (CaF2), which has a significantly higher transparency than usual lens materials such as quartz glass at these short wavelengths and therefore partially or even wholly replaces these materials.
In order to reduce such polarization-dependent effects, additional methods are provided in some projection exposure apparatuses of this kind. For example, the linearly polarized projection light generated by the laser may be converted into circularly polarized light or even into unpolarized light, as is described in U.S. Pat. No. 6,535,273.
U.S. Pat. No. 6,191,880 discloses an illumination system including a masking objective, referred to generally in that document as a relay and field optical system, and a polarizer which converts the polarization direction of incident linearly and homogeneously polarized light into radially polarized light practically without loss. For this purpose the polarizer includes a plurality of hexagonal birefringent elements which form a honeycomb arrangement and ensure the radial polarization state through position-dependent rotation of the polarization direction. The radial polarization has the advantage that disturbances resulting from polarization-dependent reflections on the light-sensitive layer to be exposed are reduced.
However, in projection exposure apparatuses whose projection lenses comprise a beam-splitter cube having a polarization-selective beam splitting layer, it may be advantageous to direct linearly and homogeneously polarized projection light on the beam splitting layer. This allows to keep light losses at the beam splitting layer low. It has also been found that in certain cases, e.g. when projecting reticles containing particularly fine structures, the use of polarized light produces higher contrast in the image of the reticle. For this reason one often attempts to preserve the linear polarization state produced by the laser in the illumination system as good as possible. This entails that disturbances of the polarization distribution which may be caused, for example, by birefringence, have to be reduced. However, the complexity and cost associated with such a reduction are relatively high.
From U.S. Pat. No. 5,815,247 an illumination system for a projection exposure apparatus is known which includes an aperture plate which is rotatable about the optical axis. The aperture plate includes two pairs of apertures which are filled by polarization filters in the form of polarizing films. The two pairs differ in the orientation of the polarization filters. A further large-area polarization filter is arranged upstream of the aperture plate, relative to which polarization filter the polarization filters in the apertures can be aligned. With this known arrangement, different pairs of apertures are used for successive exposures.
U.S. Pat. No. 5,748,369 discloses a polarization-selective beam splitter for use in liquid-crystal projectors. The polarization-selective beam splitter consists of thin glass plates provided on both sides with polarization-selective beam splitting layers. The glass plates are arranged one behind the other in a staggered formation in the propagation direction of the light.